1. Field of the Invention
The present invention relates generally to transmission of USB messages. More particularly, the present invention relates to controlling the transmission of USB messages over a data network between a USB device and a plurality of host computers.
2. Description of the Related Art
Universal Serial Bus (“USB”) has become a standard for connecting peripherals to computers. Most computers have USB ports allowing the connection of USB-enabled peripherals such as mice, keyboards, printers, scanners, digital cameras, webcams, modems, speakers, telephones, storage devices, flash card readers, network interfaces, and other devices. One benefit of USB is that any USB-enabled device (“USB device”) can be connected to its host computer using the same, standardized USB connection.
Regardless of the particular USB device, the USB device can be plugged into the host computer simply by connecting a USB cable between the device and the host computer. When the USB device is connected, the operating system of the host computer can often automatically detect it. Driver software can be easily installed on the host computer to interact with the USB device.
For host computers with fewer USB ports than desired USB devices, a USB hub can be connected to provide additional USB connections. Conventional USB hubs, such as a 4-port hub, can be plugged into the USB port on the host computer, enabling the connection of 4 USB devices to the single USB host port. USB hubs can be chained together to provide additional USB ports as desired and share the bandwidth available on the root port.
There are generally four types or modes of data transfer that can occur between the USB device and the computer: (1) Control, (2) Interrupt, (3) Bulk and (4) Isochronous. Control mode is used to enumerate (identify) a device and is often used for out of band device control such as accessing control registers on the device. Interrupt mode can be used for a device such as a mouse or keyboard that sends data at a lower bandwidth and speed. Bulk mode is used for devices such as printers which receive data in larger packets. In one example, a block of data is sent to the printer in 64-byte packets. Isochronous mode is used for streaming devices such as speakers. Data streams between the USB device and the computer.
A USB cable connects a USB device and host computer in a point-to-point manner. The USB cable limits the physical distance and connectivity between a USB device and the host. Individual USB cables are generally manufactured in lengths on the order of several meters. With USB hubs, USB devices can be connected to a host computer up to about 30 meters away from the host. However, the distance between the USB device and the host it communicates with is still limited to the physical distance of the USB cables and hubs.
Because of the physical limitation of the USB cable, attempts to connect USB devices remotely with respect to a host have been problematic. One example of a USB device which has not been effectively connected to the host as a remote device is a USB-enabled multi-function printer. Print servers exist that use an LPR protocol and provide an Ethernet connection to the printer. However, many print servers preclude the bi-directional protocol of the multi-function printer. The print server only provides the print function of the printer and eliminates the enhanced functions of the printer. Also, when the host computer is connected by virtual private network (VPN) in a corporate environment, the Ethernet enabled printer is not available to print from the host computer. In the past, the only way to access the multiple functions of the printer was to disconnect the printer from the VPN and connect the printer to the host computer by a USB cable. Then, to access the printer via the VPN, the printer had to be disconnected from USB and re-connected to the VPN.
With the advent of multiple hosts, it is desirable to share common peripherals. However, an additional limitation of USB is the inability to share USB devices among two or more hosts. Conventional techniques for connecting USB devices to hosts have only provided for one-to-one connections: one USB device to one host. Logical sharing of USB devices among two or more hosts has not been possible. The only way to arguably share a USB device has been is in the physical sense—the USB cable connecting the USB device to a first host is physically unplugged from the first host, moved into the necessary proximity with a second host, and then plugged into the second host.
What is needed is a scheme for eliminating the physical distance limitation of USB, enabling the remote connection and communication of a USB device with a host, and enabling the sharing of a USB device or devices by multiple hosts.